Turbidity Current - Examples of Turbidity Currents

Examples of Turbidity Currents

• Within minutes after the 1929 Grand Banks earthquake occurred off the coast of Newfoundland, transatlantic telephone cables began breaking sequentially, farther and farther downslope, away from the epicenter. Twelve cables were snapped in a total of 28 places. Exact times and locations were recorded for each break. Investigators suggested that a 60-mile-per-hour (100 km/h) submarine landslide or turbidity current of water saturated sediments swept 400 miles (600 km) down the continental slope from the earthquake’s epicenter, snapping the cables as it passed. Subsequent research of this event have shown that continental slope sediment failures mostly occurred below 650 meter water depth. The slumping that occurred in shallow waters (5–25 meters) passed down slope into turbidity currents that evolved ignitively. The turbidity currents had sustained flow for many hours due to the delayed retrogressive failure and transformation of debris flows into turbidity currents through hydraulic jumps.

• The Cascadia subduction zone, off the northwestern coast of North America, has a record of earthquake triggered turbidites that is well-correlated to other evidence of earthquakes recorded in coastal bays and lakes during the Holocene. Forty–one Holocene turbidity currents have been correlated along all or part of the approximately 1000 km long plate boundary stretching from northern California to mid-Vancouver island. The correlations are based on radiocarbon ages and subsurface stratigraphic methods. The inferred recurrence interval of Cascadia great earthquakes is approximately 500 years along the northern margin, and approximately 240 years along the southern margin.

• Taiwan is a hot spot for submarine turbidity currents as there are large amounts of sediment suspended in rivers, and it is seismically active, thus large accumulation of seafloor sediments and earthquake triggering. During the 2006 Pingtung earthquake off SW Taiwan, eleven submarine cables across the Kaoping canyon and Manila Trench were broken in sequence from 1500 to 4000 m deep, as a consequence of the associated turbidity currents. From the timing of each cable break the velocity of the current was determined to have a positive relationship with bathymetric slope. Current velocities were 20 m·s-1 on the steepest slopes and 3.7 m·s-1 on the shallowest slopes.